Field of the Invention
The invention relates to a method and apparatus for driving at least one transistor operated in saturation wherein a control voltage and a cut-off voltage are applied to the base of the transistor for causing the transistor to conduct current and cut-off respectively, and wherein the voltage applied to the base of the transistor is switched from the control voltage to the cut-off voltage for the purpose of shutting off the transistor.
Power transistors are frequently operated in the saturated state, for example, when they are driven as switching transistors. During this saturated or overdriven state, the so-called "collector diode" of the transistor is poled in the conduction direction. As a result, the charge carrier density is increased at the collector-side edge of the base zone and the number of charge carriers stored in the base is considerably larger than in the unsaturated state.
When the transistor is switched or cut-off, these excess charge carriers must necessarily flow off first. The level of the collector current, therefore, remains unchanged until the density of the charge carriers at the collector barrier layer has dropped off to the point at which the collector diode is again cut-off. Thus, only after a time interval which is called the storage or delay time does the collector current begin to recede, at which time it then quickly approaches its static residual current. This means, of course, that a delay occurs in the shutting-off of a transistor operated in saturation.
One particular circuit for driving a power transistor operated in saturation is presently being marketed wherein the transistor of a driver stage is connected in series with the base of the power transistor, and the base of the power transistor is connected via a further resistor to a terminal to which a cut-off voltage is applied. In this circuit, this power transistor is brought into conduction and carries the load current when the transistor of the driver stage is driven or conducting. When the transistor of the driver stage is cut-off or non-conducting the cut-off voltage is applied to the base of the power transistor via the terminal thereby shutting-off the power transistor. At this time, a negative base current flows out of the power transistor, whereby the latter is first desaturated and then shut-off. In order to obtain rapid shut-off with low losses, the resistor connecting the cut-off voltage terminal to the base of the power transistor must have a low resistance. As a result, a large part of the control current flowing via the transistor of the driver stage during conduction of the power transistor flows to the cut-off voltage terminal. This, in turn, significantly increases the power required for operation.
A further circuit for driving a power transistor operated in saturation is also being marketed wherein the base of the power transistor is connected, on the one hand, via a resistor, to a terminal to which a control voltage is being applied and, on the other hand, via the collector-emitter path of a further transistor, to a terminal to which a cut-off voltage is being applied. This further transistor is called a shut-off or depletion transistor. In this circuit, the power transistor becomes conducting and carries the load current when the shut-off transistor is cut-off. During this time, the base current flows through the resistor connecting the base of the power transistor to the control voltage terminal. To shut-off the power transistor, the shut-off transistor is driven into conduction and thereby, the cut-off voltage is applied to the base of the power transistor. Thus, a negative current flows out of the base of the power transistor, whereby the latter is first desaturated and then switched off. During the desaturation and shut-off phase of the power transistor, the shut-off transistor carries a current, to which are added the negative base current and the current which flows through the resistor connecting the base of the power transistor to the control voltage terminal. Thus, in this circuit, the shut-off transistor must be designed and driven so that it can carry the negative base current, which may be several times larger than the positive base current. In addition, control power is unnecessarily consumed during the entire cut-off time of the power transistor, as the shut-off transistor remains driven and, therefore, control current flows via the series resistor.
Another circuit for driving a power transistor is being marketed wherein a driven stage includes a switch in the form of a push-pull circuit comprised of transistors for switching a control voltage and a cut-off voltage to and from the base of the power transistor. In this circuit, the power transistor becomes conducting and carries the load current when the appropriate transistor of the push-pull circuit is conducting and the other one is cut-off. For shutting off the power transistor, the conducting transistor of the push-pull circuit is cut-off and the heretofore cut-off transistor is driven, whereby the latter transistor carries the negative base current during the desaturation and shutting-off phase. This circuit has the advantage that control current flows only during the conduction phase of the power transistor and, therefore, control power is not consumed unnecessarily. It has the disadvantage, however, that the transistor of the push-pull circuit which is being driven during the shutting off of the power transistor carries the negative base current during the entire storage time of the power transistor and, hence, must be driven and designed accordingly. It may be further necessary with this circuit for a defined shutting-off of the power transistor to design the circuit such that the negative base current rises with a defined slope or rate of change (di/dt), which adds to the cost of the circuit.
It is an object of the present invention to provide a method and apparatus for driving a transistor such that the transistor is shut off with low losses, with a minimum of control power and in a defined time interval.